Methods for coating a metal substrate and related coated metal substrates

ABSTRACT

Disclosed are methods for treating metal substrates, including ferrous substrates, such as cold rolled steel and electrogalvanized steel. The methods include depositing an electropositive metal onto at least a portion of the substrate, and then contacting the substrate with a pretreatment composition that is substantially free of crystalline phosphates and chromates. The present invention also relates to coated substrates produced thereby.

FIELD OF THE INVENTION

The present invention relates to methods for coating a metal substrate, including ferrous substrates, such as cold rolled steel and electrogalvanized steel. The present invention also relates to coated metal substrates.

BACKGROUND INFORMATION

The use of protective coatings on metal substrates for improved corrosion resistance and paint adhesion is common. Conventional techniques for coating such substrates include techniques that involve pretreating the metal substrate with a phosphate conversion coating and chrome-containing rinses. The use of such phosphate and/or chromate-containing compositions, however, imparts environmental and health concerns.

As a result, chromate-free and/or phosphate-free pretreatment compositions have been developed. Such compositions are generally based on chemical mixtures that in some way react with the substrate surface and bind to it to form a protective layer. For example, pretreatment compositions based on a group IIIB or IVB metal compound have recently become more prevalent. In some cases, it has been proposed to include copper in such compositions to improve the corrosion resisting properties of the composition. The corrosion resistance capability of these pretreatment compositions, however, even when copper is included, has generally been significantly inferior to conventional phosphate and/or chromium containing pretreatments. Moreover, the inclusion of copper in such compositions can result in the discoloration of some subsequently applied coatings, such as certain electrodeposited coatings, particularly non-black coatings. In addition, inclusion of copper in the pretreatment composition can make it more difficult to maintain the proper composition of materials in the pretreatment bath, as copper tends to deposit onto the metal surface at a rate different from the other metals in the composition.

As a result, it would be desirable to provide methods for treating a metal substrate that overcome at least some of the previously described drawbacks of the prior art, including the environmental drawbacks associated with the use of chromates and/or phosphates. Moreover, it would be desirable to provide methods for treating metal substrate that, in at least some cases, imparts corrosion resistance properties that are equivalent to, or even superior to, the corrosion resistance properties impart through the use of phosphate conversion coatings. It would also be desirable to provide related coated metal substrates.

SUMMARY OF THE INVENTION

In certain respects, the present invention is directed to methods for treating a metal substrate comprising: (a) depositing an electropositive metal onto at least a portion of the substrate, and then (b) contacting the substrate with a pretreatment composition that is substantially free of zinc phosphate and chromate.

In still other respects, the present invention is directed to methods for treating a metal substrate comprising: (a) contacting the substrate with a plating solution of a soluble metal salt, wherein the metal comprises an electropositive metal, and then subsequently (b) contacting the substrate with a pretreatment composition that is substantially free of zinc phosphate and chromate.

The present invention is also directed to substrates treated thereby.

DETAILED DESCRIPTION OF THE INVENTION

For purposes of the following detailed description, it is to be understood that the invention may assume various alternative variations and step sequences, except where expressly specified to the contrary. Moreover, other than in any operating examples, or where otherwise indicated, all numbers expressing, for example, quantities of ingredients used in the specification and claims are to be understood as being modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard variation found in their respective testing measurements.

Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.

In this application, the use of the singular includes the plural and plural encompasses singular, unless specifically stated otherwise. In addition, in this application, the use of “or” means “and/or” unless specifically stated otherwise, even though “and/or” may be explicitly used in certain instances.

As previously mentioned, certain embodiments of the present invention are directed to methods for treating a metal substrate. Suitable metal substrates for use in the present invention include those that are often used in the assembly of automotive bodies, automotive parts, and other articles, such as small metal parts, including fasteners, i.e., nuts, bolts, screws, pins, nails, clips, buttons, and the like. Specific examples of suitable metal substrates include, but are not limited to, cold rolled steel, hot rolled steel, steel coated with zinc metal, zinc compounds, or zinc alloys, such as electrogalvanized steel, hot-dipped galvanized steel, galvanealed steel, and steel plated with zinc alloy. Also, aluminum alloys, aluminum plated steel and aluminum alloy plated steel substrates may be used. Other suitable non-ferrous metals include copper and magnesium, as well as alloys of these materials. Moreover, the bare metal substrate being coating by the methods of the present invention may be a cut edge of a substrate that is otherwise treated and/or coated over the rest of its surface. The metal substrate coated in accordance with the methods of the present invention may be in the form of, for example, a sheet of metal or a fabricated part.

The substrate to be treated in accordance with the methods of the present invention may first be cleaned to remove grease, dirt, or other extraneous matter. This is often done by employing mild or strong alkaline cleaners, such as are commercially available and conventionally used in metal pretreatment processes. Examples of alkaline cleaners suitable for use in the present invention include Chemkleen 163, Chemkleen 177, and Chemkleen 490MX, each of which are commercially available from PPG Industries, Inc. Such cleaners are often followed and/or preceded by a water rinse.

As indicated, in certain methods of the present invention, an “electropositive metal” is deposited on at least a portion of the substrate. As used herein, the term “electropositive metal” refers to metals that are more electropositive than the metal substrate. This means that, for purposes of the present invention, the term “electropositive metal” encompasses metals that are less easily oxidized than the metal of the metal substrate. As will be appreciated by those skilled in the art, the tendency of a metal to be oxidized is called the oxidation potential, is expressed in volts, and is measured relative to a standard hydrogen electrode, which is arbitrarily assigned an oxidation potential of zero. The oxidation potential for several elements is set forth in the table below. An element is less easily oxidized than another element if it has a voltage value, E*, in the following table, that is greater than the element to which it is being compared.

Element Half-cell reaction Voltage, E* Potassium K⁺ + e → K −2.93 Calcium Ca²⁺ + 2e → Ca −2.87 Sodium Na⁺ + e → Na −2.71 Magnesium Mg²⁺ + 2e → Mg −2.37 Aluminum Al³⁺ + 3e → Al −1.66 Zinc Zn²⁺ + 2e → Zn −0.76 Iron Fe²⁺ + 2e → Fe −0.44 Nickel Ni²⁺ + 2e → Ni −0.25 Tin Sn²⁺ + 2e → Sn −0.14 Lead Pb²⁺ + 2e → Pb −0.13 Hydrogen 2H⁺ + 2e → H₂ −0.00 Copper Cu²⁺ + 2e → Cu 0.34 Mercury Hg₂ ²⁺ + 2e → 2Hg 0.79 Silver Ag⁺ + e → Ag 0.80 Gold Au³⁺ + 3e → Au 1.50

Thus, as will be apparent, when the metal substrate comprises one of the materials listed earlier, such as cold rolled steel, hot rolled steel, steel coated with zinc metal, zinc compounds, or zinc alloys, hot-dipped galvanized steel, galvanealed steel, steel plated with zinc alloy, aluminum alloys, aluminum plated steel, aluminum alloy plated steel, magnesium and magnesium alloys, suitable electropositive metals for deposition thereon in accordance with the present invention include, for example, nickel, copper, silver, and gold, as well mixtures thereof.

In addition, as indicated, in certain methods of the present invention, the substrate having the electropositive metal deposited thereon is then contacted with a pretreatment composition. As used herein, the term “pretreatment composition” refers to a composition that upon contact with the substrate, reacts with and chemically alters the substrate surface and binds to it to form a protective layer.

As indicated, in certain embodiments of the present invention, an electropositive metal is first deposited onto the substrate. Any suitable technique may be used to accomplish this deposition, however, in certain embodiments, the deposition is accomplished without the use of electric current. In particular, in certain embodiments, the electropositive metal is deposited by contacting the substrate with a plating solution of a soluble metal salt, such as a soluble copper salt, wherein the metal of the substrate dissolves while the metal in the solution, such as copper, is plated out onto the substrate surface.

The plating solution referenced above is often an aqueous solution of a water soluble metal salt. In certain embodiments of the present invention, the water soluble metal salt is a water soluble copper compound. Specific examples of water soluble copper compounds, which are suitable for use in the present invention include, but are not limited to, copper cyanide, copper potassium cyanide, copper sulfate, copper nitrate, copper pyrophosphate, copper thiocyanate, disodium copper ethylenediaminetetraacetate tetrahydrate, copper bromide, copper oxide, copper hydroxide, copper chloride, copper fluoride, copper gluconate, copper citrate, copper lauroyl sarcosinate, copper formate, copper acetate, copper propionate, copper butyrate, copper lactate, copper oxalate, copper phytate, copper tartarate, copper malate, copper succinate, copper malonate, copper maleate, copper benzoate, copper salicylate, copper aspartate, copper glutamate, copper fumarate, copper glycerophosphate, sodium copper chlorophyllin, copper fluorosilicate, copper fluoroborate and copper iodate, as well as copper salts of carboxylic acids in the homologous series formic acid to decanoic acid, copper salts of polybasic acids in the series oxalic acid to suberic acid, and copper salts of hydroxycarboxylic acids, including glycolic, lactic, tartaric, malic and citric acids.

When copper ions supplied from such a water-soluble copper compound are precipitated as an impurity in the form of copper sulfate, copper oxide, etc., it may be preferable to add a complexing agent that suppresses the precipitation of copper ions, thus stabilizing them as a copper complex in the solution.

In certain embodiments, the copper compound is added as a copper complex salt such as K₃Cu(CN)₄ or Cu-EDTA, which can be present stably in the plating solution on its own, but it is also possible to form a copper complex that can be present stably in the plating solution by combining a complexing agent with a compound that is difficultly soluble on its own. Examples thereof include a copper cyanide complex formed by a combination of CuCN and KCN or a combination of CuSCN and KSCN or KCN, and a Cu-EDTA complex formed by a combination of CuSO₄ and EDTA.2Na.

With regard to the complexing agent, a compound that can form a complex with copper ions can be used; examples thereof include inorganic compounds, such as cyanide compounds and thiocyanate compounds, and polycarboxylic acids, and specific examples thereof include ethylenediaminetetraacetic acid, salts of ethylenediaminetetraacetic acid, such as dihydrogen disodium ethylenediaminetetraacetate dihydrate, aminocarboxylic acids, such as nitrilotriacetic acid and iminodiacetic acid, oxycarboxylic acids, such as citric acid and tartaric acid, succinic acid, oxalic acid, ethylenediaminetetramethylenephosphonic acid, and glycine.

In certain embodiments, the electropositive metal, such as copper, is included in the plating solution in an amount of at least 1 part per million (“ppm”), such as at least 50 ppm, or, in some cases, at least 100 ppm of total metal (measured as elemental metal). In certain embodiments, the electropositive metal, such as copper, is included in the plating solution in an amount of no more than 5,000 ppm, such as no more than 1,000 ppm, or, in some cases, no more than 500 ppm of total metal (measured as elemental metal). The amount of electropositive metal in the plating solution can range between any combination of the recited values inclusive of the recited values.

In addition to the water soluble metal salt and optional complexing agent, the plating solution utilized in certain embodiments of the present invention may also include other additives. For example, a stabilizer, such as 2-mercaptobenzothiazole, may be used. Other optional materials include surfactants that function as defoamers or substrate wetting agents. Anionic, cationic, amphoteric, or nonionic surfactants may be used. Compatible mixtures of such materials are also suitable. Defoaming surfactants are often present at levels up to 1 percent, such as up to 0.1 percent by volume, and wetting agents are often present at levels up to 2 percent, such as up to 0.5 percent by volume, based on the total volume of the solution.

In certain embodiments, the aqueous plating solution utilized in certain embodiments of the present invention has a pH at application of less than 7, in some cases the pH is within the range of 1 to 4, such as 1.5 to 3.5. In certain embodiments, the pH of the solution is maintained through the inclusion of an acid. The pH of the solution may be adjusted using mineral acids, such as hydrofluoric acid, fluoroboric acid and phosphoric acid, including mixtures thereof; organic acids, such as lactic acid, acetic acid, citric acid, sulfamic acid, or mixtures thereof; and water soluble or water dispersible bases, such as sodium hydroxide, ammonium hydroxide, ammonia, or amines such as triethylamine, methylethyl amine, or mixtures thereof.

The plating solution may be brought into contact with the substrate by any of a variety of techniques, including, for example, dipping or immersion, spraying, intermittent spraying, dipping followed by spraying, spraying followed by dipping, brushing, or roll-coating. In certain embodiments, a dipping or immersion technique is used and the solution, when applied to the metal substrate, is at a temperature ranging from 60 to 185° F. (15 to 85° C.). The contact time is often from 10 seconds to five minutes, such as 30 seconds to 2 minutes. After removal of the substrate from the plating solution, the substrate may, if desired, be rinsed with water and dried.

In certain embodiments, the residue of the plating solution, i.e., the electropositive metal, is present on the substrate in an amount ranging from 1 to 1000 milligrams per square meter (mg/m²), such as 10 to 400 mg/m². The thickness of the residue of the plating solution can vary, but it is generally very thin, often having a thickness of less than 1 micrometer, in some cases it is from 1 to 500 nanometers, and, in yet other cases, it is 10 to 300 nanometers.

As previously indicated, in certain methods of the present invention, following deposition of the electropositive metal, the metal substrate is contacted with a pretreatment composition. Any pretreatment composition may be used in accordance with the methods of the present invention, so long as the composition is substantially free of crystalline phosphates and chromates.

In certain embodiments of the present invention, the pretreatment composition comprises a group IIIB and/or IVB metal compound. Indeed, it was a surprising and unexpected discovery of the present invention that the corrosion resistance capability resulting from the use of a coating method utilizing a pretreatment composition comprising a group IIIB and/or IVB metal compound can be significantly improved by practicing the methods of the present invention. Indeed, it has been seen that the improvement can be so dramatic that, in at least some cases, the corrosion resistance performance resulting from the use of such a treatment method is equivalent to that achieved through the use of a crystalline phosphate conversion coating.

In certain embodiments, the pretreatment composition comprises a carrier, often an aqueous medium, so that the composition is in the form of a solution or dispersion of a group IIIB or IVB metal compound in the carrier. In these embodiments, the solution or dispersion may be brought into contact with the substrate by any of a variety of known techniques, such as dipping or immersion, spraying, intermittent spraying, dipping followed by spraying, spraying followed by dipping, brushing, or roll-coating. In certain embodiments, the solution or dispersion when applied to the metal substrate is at a temperature ranging from 60 to 150° F. (15 to 65° C.). The contact time is often from 10 seconds to five minutes, such as 30 seconds to 2 minutes.

As used herein, the term “group IIIB and/or IVB metal” refers to an element that is in group IIIB or group IVB of the CAS Periodic Table of the Elements as is shown, for example, in the Handbook of Chemistry and Physics, 63^(rd) edition (1983). Where applicable, the metal themselves may be used. In certain embodiments, a group IIIB and/or IVB metal compound is used. As used herein, the term “group IIIB and/or IVB metal compound” refers to compounds that include at least one element that is in group IIIB or group IVB of the CAS Periodic Table of the Elements.

In certain embodiments, the group IIIB and/or IVB metal compound used in the pretreatment composition is a compound of zirconium, titanium, hafnium, yttrium, cerium, or a mixture thereof. Suitable compounds of zirconium include, but are not limited to, hexafluorozirconic acid, alkali metal and ammonium salts thereof, ammonium zirconium carbonate, zirconyl nitrate, zirconium carboxylates and zirconium hydroxy carboxylates, such as hydrofluorozirconic acid, zirconium acetate, zirconium oxalate, ammonium zirconium glycolate, ammonium zirconium lactate, ammonium zirconium citrate, and mixtures thereof. Suitable compounds of titanium include, but are not limited to, fluorotitanic acid and its salts. A suitable compound of hafnium includes, but is not limited to, hafnium nitrate. A suitable compound of yttrium includes, but is not limited to, yttrium nitrate. A suitable compound of cerium includes, but is not limited to, cerous nitrate.

In certain embodiments, the group IIIB and/or IVB metal compound is present in the pretreatment composition in an amount of at least 10 ppm metal, such as at least 100 ppm metal, or, in some cases, at least 150 ppm metal. In certain embodiments, the group IIIB and/or IVB metal compound is present in the pretreatment composition in an amount of no more than 5000 ppm metal, such as no more than 300 ppm metal, or, in some cases, no more than 250 ppm metal. The amount of group IIIB and/or IVB metal in the pretreatment composition can range between any combination of the recited values inclusive of the recited values. The pH of the pretreatment composition often ranges from 2.0 to 7.0, such as 3.5 to 5.5. The pH of the pretreatment composition may be adjusted using, for example, any of the acids and bases identified earlier with respect to the plating solution.

In some embodiments, the pretreatment composition may be a silane or a non-crystalline phosphate, such as iron phosphate, containing pretreatment composition. Suitable silane containing pretreatment compositions include, but are not limited to, certain commercially available products, such as Silquest A-1100 Silane, which is described in the Examples herein and which is commercially available from Momentive Performance Materials. Suitable non-crystalline phosphate containing pretreatment composition include pretreatment composition that comprise, iron phosphate, manganese phosphate, calcium phosphate, magnesium phosphate, cobalt phosphate, or an organophosphate and/or organophosphonate, such as is disclosed in U.S. Pat. No. 5,294,265 at col. 1, line 53 to col. 3, line 12 and U.S. Pat. No. 5,306,526 at col. 1, line 46 to col. 3, line 8, the cited portions of which being incorporated herein by reference. Suitable non-crystalline phosphate containing pretreatment compositions are commercially available, such as Chemfos® 158 and Chemfos® 51, which are iron phosphate pretreatment compositions commercially available from PPG Industries, Inc.

In certain embodiments, the pretreatment composition also comprises an electropositive metal, such as copper. The source of electropositive metal, such as copper, in the pretreatment composition may comprise, for example, any of the materials described earlier with respect to the plating solution. In certain embodiments, the electropositive metal is included in such pretreatment compositions in an amount of at least 1 ppm, such as at least 5 ppm, or in some cases, at least 10 ppm of total metal (measured as elemental metal). In certain embodiments, the electropositive metal is included in such pretreatment compositions in an amount of no more than 500 ppm, such as no more than 100 ppm, or in some cases, no more than 50 ppm of total metal (measured as elemental metal).

In certain embodiments, the pretreatment composition comprises a resinous binder. Suitable resins include reaction products of one or more alkanolamines and an epoxy-functional material containing at least two epoxy groups, such as those disclosed in U.S. Pat. No. 5,653,823. In some cases, such resins contain beta hydroxy ester, imide, or sulfide functionality, incorporated by using dimethylolpropionic acid, phthalimide, or mercaptoglycerine as an additional reactant in the preparation of the resin. Alternatively, the reaction product is that of the diglycidyl ether of Bisphenol A (commercially available from Shell Chemical Company as EPON 880), dimethylol propionic acid, and diethanolamine in a 0.6 to 5.0:0.05 to 5.5:1 mole ratio. Other suitable resinous binders include water soluble and water dispersible polyacrylic acids as disclosed in U.S. Pat. Nos. 3,912,548 and 5,328,525; phenol formaldehyde resins as described in U.S. Pat. No. 5,662,746; water soluble polyamides such as those disclosed in WO 95/33869; copolymers of maleic or acrylic acid with allyl ether as described in Canadian patent application 2,087,352; and water soluble and dispersible resins including epoxy resins, aminoplasts, phenol-formaldehyde resins, tannins, and polyvinyl phenols as discussed in U.S. Pat. No. 5,449,415.

In these embodiments of the present invention, the resinous binder is present in the pretreatment composition in an amount of 0.005 percent to 30 percent by weight, such as 0.5 to 3 percent by weight, based on the total weight of the ingredients in the composition.

In other embodiments, however, the pretreatment composition is substantially free or, in some cases, completely free of any resinous binder. As used herein, the term “substantially free”, when used with reference to the absence of resinous binder in the pretreatment composition, means that any resinous binder is present in the pretreatment composition in an amount of less than 0.005 percent by weight. As used herein, the term “completely free” means that there is no resinous binder in the pretreatment composition at all.

The pretreatment composition may optionally contain other materials, such as nonionic surfactants and auxiliaries conventionally used in the art of pretreatment. In an aqueous medium, water dispersible organic solvents, for example, alcohols with up to about 8 carbon atoms, such as methanol, isopropanol, and the like, may be present; or glycol ethers such as the monoalkyl ethers of ethylene glycol, diethylene glycol, or propylene glycol, and the like. When present, water dispersible organic solvents are typically used in amounts up to about ten percent by volume, based on the total volume of aqueous medium.

Other optional materials include surfactants that function as defoamers or substrate wetting agents, such as those materials and amounts described earlier with respect to the plating solution.

In certain embodiments, the pretreatment composition also comprises a reaction accelerator, such as nitrite ions, nitro-group containing compounds, hydroxylamine sulfate, persulfate ions, sulfite ions, hyposulfite ions, peroxides, iron (III) ions, citric acid iron compounds, bromate ions, perchlorinate ions, chlorate ions, chlorite ions as well as ascorbic acid, citric acid, tartaric acid, malonic acid, succinic acid and salts thereof. Specific examples of suitable materials and their amounts are described in United States Patent Application Publication No. 2004/0163736 A1 at [0032] to [0041], the cited portion of which being incorporated herein by reference.

In certain embodiments, the pretreatment composition also comprises a filler, such as a siliceous filler. Non-limiting examples of suitable fillers include silica, mica, montmorillonite, kaolinite, asbestos, talc, diatomaceous earth, vermiculite, natural and synthetic zeolites, cement, calcium silicate, aluminum silicate, sodium aluminum silicate, aluminum polysilicate, alumina silica gels, and glass particles. In addition to the siliceous fillers other finely divided particulate substantially water-insoluble fillers may also be employed. Examples of such optional fillers include carbon black, charcoal, graphite, titanium oxide, iron oxide, copper oxide, zinc oxide, antimony oxide, zirconia, magnesia, alumina, molybdenum disulfide, zinc sulfide, barium sulfate, strontium sulfate, calcium carbonate, and magnesium carbonate.

As indicated, in certain embodiments, the pretreatment composition is substantially or, in some cases, completely free of chromate and/or heavy metal phosphate. As used herein, the term “substantially free” when used in reference to the absence of chromate and/or heavy metal phosphate, such as zinc phosphate, in the pretreatment composition means that these substances are not present in the composition to such an extent that they cause a burden on the environment. That is, they are not substantially used and the formation of sludge, such as zinc phosphate, formed in the case of using a treating agent based on zinc phosphate, is eliminated.

Moreover, in certain embodiments, the pretreatment composition is substantially free, or, in some cases, completely free of any organic materials. As used herein, the term “substantially free”, when used with reference to the absence of organic materials in the composition, means that any organic materials are present in the composition, if at all, as an incidental impurity. In other words, the presence of any organic material does not affect the properties of the composition. As used herein, the term “completely free” means that there is no organic material in the composition at all.

In certain embodiments, the film coverage of the residue of the pretreatment coating composition generally ranges from 1 to 1000 milligrams per square meter (mg/m²), such as 10 to 400 mg/m². The thickness of the pretreatment coating can vary, but it is generally very thin, often having a thickness of less than 1 micrometer, in some cases it is from 1 to 500 nanometers, and, in yet other cases, it is 10 to 300 nanometers.

Following contact with the pretreatment solution, the substrate may be rinsed with water and dried.

It has been discovered, surprisingly, that deposition of an electropositive metal prior to contacting a metal substrate with a pretreatment composition can result, in at least some cases, in a metal substrate having corrosion resisting capabilities that are substantially improved compared to certain cases in which an electropositive metal is simply included within a pretreatment composition. Indeed, it has been seen that the improvement can be so dramatic that, in at least some cases, the corrosion resistance performance resulting from the use of such a treatment method is equivalent to that achieved through the use of a crystalline phosphate conversion coating.

In certain embodiments of the methods of the present invention, after the substrate is contacted with the pretreatment composition, it is then contacted with a coating composition comprising a film-forming resin. Any suitable technique may be used to contact the substrate with such a coating composition, including, for example, brushing, dipping, flow coating, spraying and the like. In certain embodiments, however, as described in more detail below, such contacting comprises an electrocoating step wherein an electrodepositable composition is deposited onto the metal substrate by electrodeposition.

As used herein, the term “film-forming resin” refers to resins that can form a self-supporting continuous film on at least a horizontal surface of a substrate upon removal of any diluents or carriers present in the composition or upon curing at ambient or elevated temperature. Conventional film-forming resins that may be used include, without limitation, those typically used in automotive OEM coating compositions, automotive refinish coating compositions, industrial coating compositions, architectural coating compositions, coil coating compositions, and aerospace coating compositions, among others.

In certain embodiments, the coating composition comprises a thermosetting film-forming resin. As used herein, the term “thermosetting” refers to resins that “set” irreversibly upon curing or crosslinking, wherein the polymer chains of the polymeric components are joined together by covalent bonds. This property is usually associated with a cross-linking reaction of the composition constituents often induced, for example, by heat or radiation. Curing or crosslinking reactions also may be carried out under ambient conditions. Once cured or crosslinked, a thermosetting resin will not melt upon the application of heat and is insoluble in solvents. In other embodiments, the coating composition comprises a thermoplastic film-forming resin. As used herein, the term “thermoplastic” refers to resins that comprise polymeric components that are not joined by covalent bonds and thereby can undergo liquid flow upon heating and are soluble in solvents.

As previously indicated, in certain embodiments, the substrate is contacted with a coating composition comprising a film-forming resin by an electrocoating step wherein an electrodepositable composition is deposited onto the metal substrate by electrodeposition. In the process of electrodeposition, the metal substrate being treated, serving as an electrode, and an electrically conductive counter electrode are placed in contact with an ionic, electrodepositable composition. Upon passage of an electric current between the electrode and counter electrode while they are in contact with the electrodepositable composition, an adherent film of the electrodepositable composition will deposit in a substantially continuous manner on the metal substrate.

Electrodeposition is usually carried out at a constant voltage in the range of from 1 volt to several thousand volts, typically between 50 and 500 volts. Current density is usually between 1.0 ampere and 15 amperes per square foot (10.8 to 161.5 amperes per square meter) and tends to decrease quickly during the electrodeposition process, indicating formation of a continuous self-insulating film.

The electrodepositable composition utilized in certain embodiments of the present invention often comprises a resinous phase dispersed in an aqueous medium wherein the resinous phase comprises: (a) an active hydrogen group-containing ionic electrodepositable resin, and (b) a curing agent having functional groups reactive with the active hydrogen groups of (a).

In certain embodiments, the electrodepositable compositions utilized in certain embodiments of the present invention contain, as a main film-forming polymer, an active hydrogen-containing ionic, often cationic, electrodepositable resin. A wide variety of electrodepositable film-forming resins are known and can be used in the present invention so long as the polymers are “water dispersible,” i.e., adapted to be solubilized, dispersed or emulsified in water. The water dispersible polymer is ionic in nature, that is, the polymer will contain anionic functional groups to impart a negative charge or, as is often preferred, cationic functional groups to impart a positive charge.

Examples of film-forming resins suitable for use in anionic electrodepositable compositions are base-solubilized, carboxylic acid containing polymers, such as the reaction product or adduct of a drying oil or semi-drying fatty acid ester with a dicarboxylic acid or anhydride; and the reaction product of a fatty acid ester, unsaturated acid or anhydride and any additional unsaturated modifying materials which are further reacted with polyol. Also suitable are the at least partially neutralized interpolymers of hydroxy-alkyl esters of unsaturated carboxylic acids, unsaturated carboxylic acid and at least one other ethylenically unsaturated monomer. Still another suitable electrodepositable film-forming resin comprises an alkyd-aminoplast vehicle, i.e., a vehicle containing an alkyd resin and an amine-aldehyde resin. Yet another anionic electrodepositable resin composition comprises mixed esters of a resinous polyol, such as is described in U.S. Pat. No. 3,749,657 at col. 9, lines 1 to 75 and col. 10, lines 1 to 13, the cited portion of which being incorporated herein by reference. Other acid functional polymers can also be used, such as phosphatized polyepoxide or phosphatized acrylic polymers as are known to those skilled in the art.

As aforementioned, it is often desirable that the active hydrogen-containing ionic electrodepositable resin (a) is cationic and capable of deposition on a cathode. Examples of such cationic film-forming resins include amine salt group-containing resins, such as the acid-solubilized reaction products of polyepoxides and primary or secondary amines, such as those described in U.S. Pat. Nos. 3,663,389; 3,984,299; 3,947,338; and 3,947,339. Often, these amine salt group-containing resins are used in combination with a blocked isocyanate curing agent. The isocyanate can be fully blocked, as described in U.S. Pat. No. 3,984,299, or the isocyanate can be partially blocked and reacted with the resin backbone, such as is described in U.S. Pat. No. 3,947,338. Also, one-component compositions as described in U.S. Pat. No. 4,134,866 and DE-OS No. 2,707,405 can be used as the film-forming resin. Besides the epoxy-amine reaction products, film-forming resins can also be selected from cationic acrylic resins, such as those described in U.S. Pat. Nos. 3,455,806 and 3,928,157.

Besides amine salt group-containing resins, quaternary ammonium salt group-containing resins can also be employed, such as those formed from reacting an organic polyepoxide with a tertiary amine salt as described in U.S. Pat. Nos. 3,962,165; 3,975,346; and 4,001,101. Examples of other cationic resins are ternary sulfonium salt group-containing resins and quaternary phosphonium salt-group containing resins, such as those described in U.S. Pat. Nos. 3,793,278 and 3,984,922, respectively. Also, film-forming resins which cure via transesterification, such as described in European Application No. 12463 can be used. Further, cationic compositions prepared from Mannich bases, such as described in U.S. Pat. No. 4,134,932, can be used.

In certain embodiments, the resins present in the electrodepositable composition are positively charged resins which contain primary and/or secondary amine groups, such as described in U.S. Pat. Nos. 3,663,389; 3,947,339; and 4,116,900. In U.S. Pat. No. 3,947,339, a polyketimine derivative of a polyamine, such as diethylenetriamine or triethylenetetraamine, is reacted with a polyepoxide. When the reaction product is neutralized with acid and dispersed in water, free primary amine groups are generated. Also, equivalent products are formed when polyepoxide is reacted with excess polyamines, such as diethylenetriamine and triethylenetetraamine, and the excess polyamine vacuum stripped from the reaction mixture, as described in U.S. Pat. Nos. 3,663,389 and 4,116,900.

In certain embodiments, the active hydrogen-containing ionic electrodepositable resin is present in the electrodepositable composition in an amount of 1 to 60 percent by weight, such as 5 to 25 percent by weight, based on total weight of the electrodeposition bath.

As indicated, the resinous phase of the electrodepositable composition often further comprises a curing agent adapted to react with the active hydrogen groups of the ionic electrodepositable resin. For example, both blocked organic polyisocyanate and aminoplast curing agents are suitable for use in the present invention, although blocked isocyanates are often preferred for cathodic electrodeposition.

Aminoplast resins, which are often the preferred curing agent for anionic electrodeposition, are the condensation products of amines or amides with aldehydes. Examples of suitable amine or amides are melamine, benzoguanamine, urea and similar compounds. Generally, the aldehyde employed is formaldehyde, although products can be made from other aldehydes, such as acetaldehyde and furfural. The condensation products contain methylol groups or similar alkylol groups depending on the particular aldehyde employed. Often, these methylol groups are etherified by reaction with an alcohol, such as a monohydric alcohol containing from 1 to 4 carbon atoms, such as methanol, ethanol, isopropanol, and n-butanol. Aminoplast resins are commercially available from American Cyanamid Co. under the trademark CYMEL and from Monsanto Chemical Co. under the trademark RESIMENE.

The aminoplast curing agents are often utilized in conjunction with the active hydrogen containing anionic electrodepositable resin in amounts ranging from 5 percent to 60 percent by weight, such as from 20 percent to 40 percent by weight, the percentages based on the total weight of the resin solids in the electrodepositable composition.

As indicated, blocked organic polyisocyanates are often used as the curing agent in cathodic electrodeposition compositions. The polyisocyanates can be fully blocked as described in U.S. Pat. No. 3,984,299 at col. 1, lines 1 to 68, col. 2, and col. 3, lines 1 to 15, or partially blocked and reacted with the polymer backbone as described in U.S. Pat. No. 3,947,338 at col. 2, lines 65 to 68, col. 3, and col. 4 lines 1 to 30, the cited portions of which being incorporated herein by reference. By “blocked” is meant that the isocyanate groups have been reacted with a compound so that the resultant blocked isocyanate group is stable to active hydrogens at ambient temperature but reactive with active hydrogens in the film forming polymer at elevated temperatures usually between 90° C. and 200° C.

Suitable polyisocyanates include aromatic and aliphatic polyisocyanates, including cycloaliphatic polyisocyanates and representative examples include diphenylmethane-4,4′-diisocyanate (MDI), 2,4- or 2,6-toluene diisocyanate (TDI), including mixtures thereof, p-phenylene diisocyanate, tetramethylene and hexamethylene diisocyanates, dicyclohexylmethane-4,4′-diisocyanate, isophorone diisocyanate, mixtures of phenylmethane-4,4′-diisocyanate and polymethylene polyphenylisocyanate. Higher polyisocyanates, such as triisocyanates can be used. An example would include triphenylmethane-4,4′,4″-triisocyanate. Isocyanate ( )-prepolymers with polyols such as neopentyl glycol and trimethylolpropane and with polymeric polyols such as polycaprolactone diols and triols (NCO/OH equivalent ratio greater than 1) can also be used.

The polyisocyanate curing agents are typically utilized in conjunction with the active hydrogen containing cationic electrodepositable resin in amounts ranging from 5 percent to 60 percent by weight, such as from 20 percent to 50 percent by weight, the percentages based on the total weight of the resin solids of the electrodepositable composition.

In certain embodiments, the coating composition comprising a film-forming resin also comprises yttrium. In certain embodiments, yttrium is present in such compositions in an amount from 10 to 10,000 ppm, such as not more than 5,000 ppm, and, in some cases, not more than 1,000 ppm, of total yttrium (measured as elemental yttrium).

Both soluble and insoluble yttrium compounds may serve as the source of yttrium. Examples of yttrium sources suitable for use in lead-free electrodepositable coating compositions are soluble organic and inorganic yttrium salts such as yttrium acetate, yttrium chloride, yttrium formate, yttrium carbonate, yttrium sulfamate, yttrium lactate and yttrium nitrate. When the yttrium is to be added to an electrocoat bath as an aqueous solution, yttrium nitrate, a readily available yttrium compound, is a preferred yttrium source. Other yttrium compounds suitable for use in electrodepositable compositions are organic and inorganic yttrium compounds such as yttrium oxide, yttrium bromide, yttrium hydroxide, yttrium molybdate, yttrium sulfate, yttrium silicate, and yttrium oxalate. Organoyttrium complexes and yttrium metal can also be used. When the yttrium is to be incorporated into an electrocoat bath as a component in the pigment paste, yttrium oxide is often the preferred source of yttrium.

The electrodepositable compositions described herein are in the form of an aqueous dispersion. The term “dispersion” is believed to be a two-phase transparent, translucent or opaque resinous system in which the resin is in the dispersed phase and the water is in the continuous phase. The average particle size of the resinous phase is generally less than 1.0 and usually less than 0.5 microns, often less than 0.15 micron.

The concentration of the resinous phase in the aqueous medium is often at least 1 percent by weight, such as from 2 to 60 percent by weight, based on total weight of the aqueous dispersion. When such compositions are in the form of resin concentrates, they generally have a resin solids content of 20 to 60 percent by weight based on weight of the aqueous dispersion.

The electrodepositable compositions described herein are often supplied as two components: (1) a clear resin feed, which includes generally the active hydrogen-containing ionic electrodepositable resin, i.e., the main film-forming polymer, the curing agent, and any additional water-dispersible, non-pigmented components; and (2) a pigment paste, which generally includes one or more pigments, a water-dispersible grind resin which can be the same or different from the main-film forming polymer, and, optionally, additives such as wetting or dispersing aids. Electrodeposition bath components (1) and (2) are dispersed in an aqueous medium which comprises water and, usually, coalescing solvents.

As aforementioned, besides water, the aqueous medium may contain a coalescing solvent. Useful coalescing solvents are often hydrocarbons, alcohols, esters, ethers and ketones. The preferred coalescing solvents are often alcohols, polyols and ketones. Specific coalescing solvents include isopropanol, butanol, 2-ethylhexanol, isophorone, 2-methoxypentanone, ethylene and propylene glycol and the monoethyl monobutyl and monohexyl ethers of ethylene glycol. The amount of coalescing solvent is generally between 0.01 and 25 percent, such as from 0.05 to 5 percent by weight based on total weight of the aqueous medium.

In addition, a colorant and, if desired, various additives such as surfactants, wetting agents or catalyst can be included in the coating composition comprising a film-forming resin. As used herein, the term “colorant” means any substance that imparts color and/or other opacity and/or other visual effect to the composition. The colorant can be added to the composition in any suitable form, such as discrete particles, dispersions, solutions and/or flakes. A single colorant or a mixture of two or more colorants can be used.

Example colorants include pigments, dyes and tints, such as those used in the paint industry and/or listed in the Dry Color Manufacturers Association (DCMA), as well as special effect compositions. A colorant may include, for example, a finely divided solid powder that is insoluble but wettable under the conditions of use. A colorant can be organic or inorganic and can be agglomerated or non-agglomerated. Colorants can be incorporated by use of a grind vehicle, such as an acrylic grind vehicle, the use of which will be familiar to one skilled in the art.

Example pigments and/or pigment compositions include, but are not limited to, carbazole dioxazine crude pigment, azo, monoazo, disazo, naphthol AS, salt type (lakes), benzimidazolone, condensation, metal complex, isoindolinone, isoindoline and polycyclic phthalocyanine, quinacridone, perylene, perinone, diketopyrrolo pyrrole, thioindigo, anthraquinone, indanthrone, anthrapyrimidine, flavanthrone, pyranthrone, anthanthrone, dioxazine, triarylcarbonium, quinophthalone pigments, diketo pyrrolo pyrrole red (“DPPBO red”), titanium dioxide, carbon black and mixtures thereof. The terms “pigment” and “colored filler” can be used interchangeably.

Example dyes include, but are not limited to, those that are solvent and/or aqueous based such as pthalo green or blue, iron oxide, bismuth vanadate, anthraquinone, perylene, aluminum and quinacridone.

Example tints include, but are not limited to, pigments dispersed in water-based or water miscible carriers such as AQUA-CHEM 896 commercially available from Degussa, Inc., CHARISMA COLORANTS and MAXITONER INDUSTRIAL COLORANTS commercially available from Accurate Dispersions division of Eastman Chemical, Inc.

As noted above, the colorant can be in the form of a dispersion including, but not limited to, a nanoparticle dispersion. Nanoparticle dispersions can include one or more highly dispersed nanoparticle colorants and/or colorant particles that produce a desired visible color and/or opacity and/or visual effect. Nanoparticle dispersions can include colorants such as pigments or dyes having a particle size of less than 150 nm, such as less than 70 nm, or less than 30 nm. Nanoparticles can be produced by milling stock organic or inorganic pigments with grinding media having a particle size of less than 0.5 mm. Example nanoparticle dispersions and methods for making them are identified in U.S. Pat. No. 6,875,800 B2, which is incorporated herein by reference. Nanoparticle dispersions can also be produced by crystallization, precipitation, gas phase condensation, and chemical attrition (i.e., partial dissolution). In order to minimize re-agglomeration of nanoparticles within the coating, a dispersion of resin-coated nanoparticles can be used. As used herein, a “dispersion of resin-coated nanoparticles” refers to a continuous phase in which is dispersed discreet “composite microparticles” that comprise a nanoparticle and a resin coating on the nanoparticle. Example dispersions of resin-coated nanoparticles and methods for making them are identified in United States Patent Application Publication 2005-0287348 A1, filed Jun. 24, 2004, U.S. Provisional Application No. 60/482,167 filed Jun. 24, 2003, and U.S. patent application Ser. No. 11/337,062, filed Jan. 20, 2006, which is also incorporated herein by reference.

Example special effect compositions that may be used include pigments and/or compositions that produce one or more appearance effects such as reflectance, pearlescence, metallic sheen, phosphorescence, fluorescence, photochromism, photosensitivity, thermochromism, goniochromism and/or color-change. Additional special effect compositions can provide other perceptible properties, such as opacity or texture. In certain embodiments, special effect compositions can produce a color shift, such that the color of the coating changes when the coating is viewed at different angles. Example color effect compositions are identified in U.S. Pat. No. 6,894,086, incorporated herein by reference. Additional color effect compositions can include transparent coated mica and/or synthetic mica, coated silica, coated alumina, a transparent liquid crystal pigment, a liquid crystal coating, and/or any composition wherein interference results from a refractive index differential within the material and not because of the refractive index differential between the surface of the material and the air.

In certain embodiments, a photosensitive composition and/or photochromic composition, which reversibly alters its color when exposed to one or more light sources, can be used in the present invention. Photochromic and/or photosensitive compositions can be activated by exposure to radiation of a specified wavelength. When the composition becomes excited, the molecular structure is changed and the altered structure exhibits a new color that is different from the original color of the composition. When the exposure to radiation is removed, the photochromic and/or photosensitive composition can return to a state of rest, in which the original color of the composition returns. In certain embodiments, the photochromic and/or photosensitive composition can be colorless in a non-excited state and exhibit a color in an excited state. Full color-change can appear within milliseconds to several minutes, such as from 20 seconds to 60 seconds. Example photochromic and/or photosensitive compositions include photochromic dyes.

In certain embodiments, the photosensitive composition and/or photochromic composition can be associated with and/or at least partially bound to, such as by covalent bonding, a polymer and/or polymeric materials of a polymerizable component. In contrast to some coatings in which the photosensitive composition may migrate out of the coating and crystallize into the substrate, the photosensitive composition and/or photochromic composition associated with and/or at least partially bound to a polymer and/or polymerizable component in accordance with certain embodiments of the present invention, have minimal migration out of the coating. Example photosensitive compositions and/or photochromic compositions and methods for making them are identified in U.S. application Ser. No. 10/892,919 filed Jul. 16, 2004, incorporated herein by reference.

In general, the colorant can be present in the coating composition in any amount sufficient to impart the desired visual and/or color effect. The colorant may comprise from 1 to 65 weight percent, such as from 3 to 40 weight percent or 5 to 35 weight percent, with weight percent based on the total weight of the compositions.

After deposition, the coating is often heated to cure the deposited composition. The heating or curing operation is often carried out at a temperature in the range of from 120 to 250° C., such as from 120 to 190° C. for a period of time ranging from 10 to 60 minutes. In certain embodiments, the thickness of the resultant film is from 10 to 50 microns.

As will be appreciated by the foregoing description, the present invention is directed to methods for coating a metal substrate comprising: (a) depositing an electropositive metal onto at least a portion of the substrate, then (b) contacting the substrate with a pretreatment composition, and then (c) depositing a coating on the substrate that is formed from a composition comprising a film-forming resin. These methods of the present invention do not include depositing a zinc phosphate or chromate-containing coating on the substrate.

As has been indicated throughout the foregoing description, the methods and coated substrates of the present invention, in certain embodiments, do not include the deposition of a heavy metal phosphate, such as zinc phosphate, or a chromate. As a result, the environmental drawbacks associated with such materials is avoided. Nevertheless, the methods of the present invention have been shown to provide coated substrates that are, in at least some cases, resistant to corrosion at a level comparable to, in some cases even superior to (as illustrated in the Examples), methods wherein such materials are used. This is a surprising and unexpected discovery of the present invention and satisfies a long felt need in the art. In addition, the methods of the present invention have been shown to avoid the discoloration of subsequently applied coatings, such as certain non-black electrodeposited coatings.

Illustrating the invention are the following examples that are not to be considered as limiting the invention to their details. All parts and percentages in the examples, as well as throughout the specification, are by weight unless otherwise indicated.

EXAMPLE 1

Cold rolled steel (CRS) panels were cleaned by spraying with a solution of Chemkleen 490MX, an alkaline cleaner available from PPG Industries, for two minutes at 120° F. After alkaline cleaning, the panels were rinsed thoroughly with deionized water. Some of the panels were then immersed in a zirconium pretreatment solution containing varying levels of copper for either two or five minutes at 120° F. The zirconium pretreatment solution was prepared by diluting hexafluorozirconic acid with water to a zirconium concentration of 175 ppm (as zirconium) and adjusting the pH to 4.5 with dilute ammonia, and adding the desired amount of copper as the copper (II) chloride dihydrate. These panels were then rinsed thoroughly with deionized water and then dried with a warm air blowoff. The other panels were immersed in an acidic solution containing various amounts of copper for two minutes at 120° F. The acid solution was prepared by diluting 198.1 grams of 85% phosphoric acid, 8.5 grams of 70% nitric acid, 16.5 grams of Triton™ X-100 (available from The Dow Chemical Company) and 11.1 grams of Triton CF-10 (available from The Dow Chemical Company) to five gallons of volume with deionized water, and then neutralizing to pH 3.0 with Chemfil Buffer (available from PPG Industries), and then adding the desired amount of copper as the copper (II) chloride dihydrate. After treatment in the acid solution, the panels were rinsed thoroughly with deionized water, and then immerse in a zirconium pretreatment solution for either two or five minutes at 120° F. as above, except that the zirconium pretreatment solution contained no copper. After pretreatment in the zirconium pretreatment solution, the panels were rinsed thoroughly with deionized water and then dried with a warm air blowoff. All pretreated panels were then electrocoated with ED 6100H, a cathodic electrocoat available from PPG Industries. The ED 6100H coating bath was prepared and coated out, and the coated panels cured, according to the manufacturer's instructions.

After coating, some panels were subjected to testing according to GM 9540-P (“Cycle B”), a cyclical corrosion test, for 60 cycles. The other panels were subjected to the Honda Salt Water Resistance test for 35 cycles. After testing, the panels were media-blasted to remove loose paint and corrosion products, and paint loss from the scribe (creep) was measured and the average calculated in millimeters for each panel. The results appear in Table I, below:

TABLE I Copper in Zirconium Copper in zirconium bath, pretreat Average creep acid bath ppm immersion time Cycle B Honda Salt No Acid Step 10 ppm 2 min 6 mm 5 mm 10 ppm  0 ppm 2 min 4 mm 5 mm No Acid Step 10 ppm 5 min 7 mm 6 mm 10 ppm  0 ppm 5 min 6 mm 5 mm No Acid Step 20 ppm 2 min 10 mm  10 mm  20 ppm  0 ppm 2 min 4 mm 3 mm No Acid Step 20 ppm 5 min 8 mm 16 mm  20 ppm  0 ppm 5 min 5 mm 8 mm

A zinc-phosphated CRS panel with a deionized water post rinse, electrocoated with the same coating, would typically have 5 to 6 mm creep after 60 cycles of Cycle B testing, and an average of 9 mm creep after 35 cycles of Honda Salt Dip testing. A zinc-phosphated panel with a zirconium-based post rinse, electrocoated with the same coating, would typically have about 3 mm creep after 60 cycles of Cycle B testing, and about 4 mm creep after 35 cycles of Honda Salt Dip testing.

EXAMPLE 2

CRS panels were cleaned by spraying with a solution of Chemkleen 490MX, an alkaline cleaner available from PPG Industries, for two minutes at 120° F. After alkaline cleaning, the panels were rinsed thoroughly with deionized water. Some of the panels were then immersed in a zirconium pretreatment solution containing varying levels of copper for two minutes at 85° F. The zirconium pretreatment solution was prepared by diluting hexafluorozirconic acid with water to a zirconium concentration of 175 ppm (as zirconium) and adjusting the pH to 4.5 with dilute ammonia, and adding the desired amount of copper as the copper(II) nitrate hemipentahydrate. These panels were then rinsed thoroughly with deionized water and then dried with a warm air blowoff. The other panels were immersed in an acidic solution containing various amounts of copper for two minutes at 120° F. The acid solution was prepared as in example 1, except that copper was added as copper(II) nitrate hemipentahydrate. After treatment in the acid solution, the panels were rinsed thoroughly with deionized water, and then immerse in a zirconium pretreatment solution for two minutes at 85° F. as above, except that the zirconium pretreatment solution contained no copper. After pretreatment in the zirconium pretreatment solution, the panels were rinsed thoroughly with deionized water and then dried with a warm air blowoff. All pretreated panels were then electrocoated with ED 6100H, a cathodic electrocoat available from PPG Industries. The ED 6100H coating bath was prepared and coated out, and the coated panels cured, according to the manufacturer's instructions.

After coating, panels were tested for corrosion resistance as in Example 1, with some panels subjected to Cycle B for 60 cycles and others to Honda Salt Water Resistance for 35 cycles, as in Example 1. After testing, the panels were media-blasted to remove loose paint and corrosion products, and paint loss from the scribe (creep) was measured and the average calculated in millimeters for each panel. The results appear in Table II, below:

TABLE II Copper in Copper in Average Creep acid bath zirconium bath Cycle B Honda Salt No acid 0 11 mm 15 mm treatment 3 10 mm 12 mm 10 12 mm  8 mm 50  7 mm  9 mm 100  6 mm 12 mm 500 22 mm 40 mm  0 ppm No copper in 10 mm 15 mm  3 ppm zirconium 12 mm 15 mm  10 ppm treatment  7 mm 10 mm  50 ppm  4 mm  6 mm 100 ppm  4 mm  5 mm 500 ppm  4 mm  6 mm

EXAMPLE 3

CRS panels were cleaned by spraying with Chemkleen 490MX, and then rinsed with deionized water, as detailed in the previous examples. Some of these panels were then treated in a copper-containing acid treatment. The acid treatment solution was prepared by diluting 39.7 grams of 96% sulfuric acid, 287 grams of 70% hydroxyacetic acid, 16.5 grams of Triton X-100 (available from The Dow Chemical Company) and 11.1 grams of Triton CF-10 (available from The Dow Chemical Company) to five gallons of volume with deionized water, and then neutralizing to pH 3.0 with Chemfil Buffer (available from PPG Industries), and then adding 50 ppm of copper as the copper sulfate pentahydrate. Panels treated in the acid step were then rinsed thoroughly with deionized water. The acid-treated panels, as well as those not treated in the acid step, were then subjected either to an iron phosphate pretreatment followed by deionized water rinsing and warm air blow-off, or a silane pretreatment with no rinse but with a subsequent oven bake for five minutes at 220° F. The iron phosphate pretreatment was Chemfos 51 (available from PPG Industries), applied at 5% v/v and pH 5.5 for one minute at 140° F. by spray application. The silane pretreatment was 2% w/v γ-aminopropyltriethoxy silane (Silquest A-1100, available from Momentive Performance Materials Inc.), adjusted to pH 8.5 with acetic acid and applied by a 30 second immersion with no subsequent rinse.

After pretreatment, all panels were electrocoated with ED 6100H, a cathodic electrocoat available from PPG Industries. The ED 6100H coating bath was prepared and coated out, and the coated panels cured, according to the manufacturer's instructions. After coating the panels were subjected to Cycle B testing for 60 cycles. After testing, the panels were media-blasted to remove loose paint and corrosion products, and creep was measured and the average calculated in millimeters for each panel. The results appear in Table III, below:

TABLE III Copper- containing acid Average Creep in Pretreatment step Cycle B Testing Silane No 25 mm Silane Yes 15 mm Iron phosphate No 23 mm Iron phosphate Yes  9 mm

EXAMPLE 4

CRS panels were cleaned by spraying with Chemkleen 490MX, and then rinsed with deionized water, as detailed in the previous examples. Some of these panels were then treated in a copper-containing acid treatment. The acid treatment solution was prepared by diluting 96% sulfuric acid to 9.3 grams per liter, adding Triton X-100 and Triton CF-10 to concentrations of 0.88 and 0.59 grams per liter, respectively, and adjusting pH to 3.0 with Chemfil Buffer. Sufficient copper sulfate pentahydrate was added to provide a concentration of 75 ppm of copper. Panels treated in the acid step were then rinsed thoroughly with deionized water. All panels were then immersed in a zirconium pretreatment solution at 120° F. for two minutes, and then thoroughly rinsed with deionized water and blown dry with warm air. The zirconium pretreatment solution was prepared by diluting hexafluorozirconic acid with water to a zirconium concentration of 175 ppm (as zirconium) and adjusting the pH to 4.5 with dilute ammonia.

After pretreatment, the panels were then electrocoated with ED 6100H, a cathodic electrocoat available from PPG Industries. The ED 6100H coating bath was prepared and coated out according to the manufacturer's instructions. However, instead of curing the electrodeposited coating, the panels were allowed to dry under ambient conditions for two minutes. Then, a crosshatch was made on each panel using a multiple-tooth cutter familiar to those skilled in the art. The crosshatched area was taped and pulled using Scotch® Filament Tape 898, available from 3M, and any adhesion loss noted. The edge of the panel was similarly taped and pulled, with no crosshatch, and any adhesion loss noted. The panels with the copper-containing acid treatment exhibited virtually no adhesion loss. The panels without the copper-containing acid treatment, in contrast, exhibited adhesion loss over most of the crosshatch area, as well as the taped area of the edge. The shiny exposed surface indicated that the adhesion loss was at the substrate-pretreatment interface.

EXAMPLE 5

Cold rolled steel panels were cleaned by spraying with a solution of Chemkleen 490MX, an alkaline cleaner available from PPG Industries, for two minutes at 120° F. After alkaline cleaning, the panels were rinsed thoroughly with deionized water. Some of the panels were then immersed in a zirconium pretreatment solution containing 10 ppm of copper for twenty minutes at 120° F. The zirconium pretreatment solution was prepared by diluting hexafluorozirconic acid with water to a zirconium concentration of 175 ppm (as zirconium) and adjusting the pH to 4.5 with dilute ammonia, and adding the desired amount of copper as the copper (II) chloride dihydrate. These panels were then rinsed thoroughly with deionized water and then dried with a warm air blowoff. The other panels were immersed in an acidic solution containing 10 ppm of copper for two minutes at 120° F. The acid solution was prepared by diluting 198.1 grams of 85% phosphoric acid, 8.5 grams of 70% nitric acid, 16.5 grams of Triton™ X-100 (available from The Dow Chemical Company) and 11.1 grams of Triton CF-10 (available from The Dow Chemical Company) to five gallons of volume with deionized water, and then neutralizing to pH 3.0 with Chemfil Buffer (available from PPG Industries), and then adding the desired amount of copper as the copper (II) chloride dihydrate. After treatment in the acid solution, the panels were rinsed thoroughly with deionized water, and then immersed in a zirconium pretreatment solution for twenty minutes at 120° F. as above, except that the zirconium pretreatment solution contained no copper. After pretreatment in the zirconium pretreatment solution, the panels were rinsed thoroughly with deionized water and then dried with a warm air blowoff. Panels from each variation were electrocoated with ED 6100H, a cathodic electrocoat available from PPG Industries. The ED 6100H coating bath was prepared and coated out, and the coated panels cured, according to the manufacturer's instructions.

After coating, some panels were subjected to testing according to GM 9540-P (“Cycle B”), a cyclical corrosion test, for 60 cycles. The other panels were subjected to the Honda Salt Water Resistance test for 35 cycles. After testing, the panels were media-blasted to remove loose paint and corrosion products, and paint loss from the scribe (creep) was measured and the average calculated in millimeters for each panel. The results appear in Table V, below:

TABLE V Example Copper in Copper in Average Creep No. acid bath zirconium bath Cycle B Honda Salt 5a No acid 10 ppm 23 mm 18 mm treatment 5b 10 ppm  0 ppm  9 mm 12 mm

It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications which are within the spirit and scope of the invention, as defined by the appended claims. 

We claim:
 1. A method for treating a metal substrate comprising: (a) contacting the metal substrate with a plating solution containing an electropositive metal in an amount from 10 to 50 ppm of total electropositive metal, measured as elemental metal, to deposit a residue of the plating solution onto at least a portion of the substrate, the residue having a thickness of from 1 to 500 nanometers; and then subsequently (b) contacting the substrate with a pretreatment composition that is substantially free of zinc phosphate, and iron phosphate and chromate and a resinous binder; and then (c) contacting the substrate with a coating composition comprising a cationic film-forming resin.
 2. The method of claim 1, wherein the electropositive metal is selected from the group consisting of nickel, copper, silver, gold and mixtures thereof.
 3. The method of claim 2, wherein the electropositive metal comprises copper.
 4. The method of claim 1, wherein the electropositive metal is a soluble metal salt.
 5. The method of claim 1, wherein the contacting with the plating solution comprises immersing the substrate in the plating solution.
 6. The method of claim 1, wherein the pretreatment composition comprises a group IIIB or IVB metal compound and is substantially free of crystalline phosphates.
 7. The method of claim 6, wherein the group IIIB and/or IVB metal compound is a compound of zirconium, titanium, hafnium, yttrium, cerium, or a mixture thereof.
 8. The method of claim 1, wherein the plating solution has a pH at application of less than
 7. 9. The method of claim 1, wherein the plating solution has a pH at application of 1 to
 4. 10. The method of claim 1, wherein the plating solution has a pH at application of 1.5 to 3.5.
 11. The method of claim 1, wherein the substrate is not contacted with an additional pretreatment composition prior to or subsequent to the (b) contacting the substrate with the pretreatment composition.
 12. A method for treating a metal substrate comprising: (a) contacting the substrate with a plating solution containing a soluble electropositive metal salt in an amount from 10 to 50 ppm of total electropositive metal, measured as elemental metal to deposit the electropositive metal on the metal substrate in an amount from 1 to 1000 mg/m²; and then subsequently (b) contacting the substrate with a pretreatment composition that is substantially free of zinc phosphate and iron phosphate and chromate and a resinous binder; and then (c) contacting the substrate with a coating composition comprising a cationic film-forming resin.
 13. The method of claim 8, wherein the electropositive metal is selected from the group consisting of nickel, copper, silver, gold and mixtures thereof.
 14. The method of claim 8, wherein the pretreatment composition comprises a group IIIB or NB metal compound and is substantially free of crystalline phosphates.
 15. The method of claim 14, wherein the group IIIB and/or IVB metal compound is a compound of zirconium, titanium, hafnium, yttrium, cerium, or a mixture thereof.
 16. The method of claim 8, wherein the substrate is not contacted with an additional pretreatment composition prior to or subsequent to the (b) contacting the substrate with the pretreatment composition. 